1. Field of the Invention
The present invention is related to a composition which serves as a bioactive coating for harvested agricultural commodities to delay ripening and control decay.
2. Description of the Prior Art
Present coatings (mostly waxes) for postharvest commodities are effective in delaying ripening, but in general, do not prevent decay. Moreover, they are under scrutiny as possible health hazards. Synthetic fungicides which have been added to the coatings to alleviate the problems of decay have recently been withdrawn from the market, and there is also public pressure to remove the petroleum-based coatings because of health and environmental concerns. A critical need therefore exists for alternatives to present coatings for agricultural commodities which are fungicidal as well as safe for the consumer and the environment.
Antagonistic yeasts have been disclosed as effective biocontrol agents for the biological control of postharvest disease (Wilson and El Ghaouth. Symposium Proceeding, Beltsville Symposium XVIII. 1993). In addition, chitosan, an animal-derived polymer, has shown potnetial an an antifungal preservative. Chitosan and its derivatives are known to form a semi-permeable film (Averbach, B. L., Film-forming capability of chitosan, in Proc. 1st Int. Conf. Chitin and Chitosan. Muzzareli and Pariser, eds. MIT, Cambridge, Mass. 1978. p. 199), to be inhibitory to a number of pathogenic fungi (Allan and Hadwiger, Exp. Mycol. 1979. vol. 3, p. 285) and to activate a number of biological processes in plant tissues, including the stimulation of chitinases, the accumulation of phytoalexins, the synthesis of proteinase inhibitors, and increased lignification (Pearce et al., Physiol. Plant Pathol. 1982, vol. 20, p. 119; Mauch et al., Plant Physiol. 1988. vol. 76, p. 607; El Ghaouth et al., Potential use of chitosan in postharvest preservation of fruits and vegetables, in Advances in Chitin and Chitosan. Brines et al., eds. Elsevier Applied Sci., London. 1992. p. 440; El Ghaouth et al., Phys. and Mol. Plant Pathol. 1994. vol. 44, pp. 417-422). The polycationic nature of chitosan is believed to provide the basis for its physico-chemical and biological functionality. When applied as a coating, chitosan controlled decay and delayed ripening of strawberry, bell pepper, tomato and cucumber by acting as a selective barrier to gas diffusion (El Ghaouth et al., in Advances in Chitin and Chitosan. 1992, supra). The control of decay by chitosan is believed to originate, in part, from its antifungal property. Indeed, in vitro studies showed that chitosan not only inhibited the radial growth of major postharvest pathogens, but also induced severe morphological alterations in Rhizopus stolonifer and Botrytis cinerea, as well as increased cellular leakage in both fungi, presumably by interfering with fungal plasma membranes (El Ghaouth et al., Mycol. Res. 1992. vol. 96, p. 769; El Ghaouth et al., Exp. Mycol. 1992. vol. 16. p. 173). Although the biological activity of chitosan is well-documented, the mechanisms that underlie its ability to function as both a fungal inhibitor and an elicitor of plant defense reactions remains unclear.